Very intensive chemo- and/or radiotherapy followed by a bone marrow transplant offers the only realistic chance of long-term disease- free survival for many patients with leukemia and certain types of disseminated solid tumors who have relapsed after standard therapy. Transplantation of the patient's own remission marrow (autologous bone marrow transplant) is now used with increasing frequency when the patient is ineligible for an allogenic bone marrow transplant for lack of a histocompatible marrow donor or for other reasons. However, retrospective studies have shown that recipients of autologous marrow grafts have a very high rate of relapse that is most likely attributable to occult clonogenic tumor cells in the remission marrow. We have recently proposed to use merocyanine 540 (MC540) mediated photosensitization to kill leukemia, lymphoma, and neuroblastoma cells in autologous marrow grafts without causing unacceptable damage to normal pluripotent hematopoietic stem cells. A phase 1/11 clinical trial of the technique is already in progress at out institution, but the underlying mechanism of the differential photosensitivity is still poorly understood. The main goal of this project is to elucidate the molecular basis of the differential affinity of merocyanine dyes for different cell types, to identify the activated oxygen species that are generated by photoexcited dye, to identify their cellular and molecular targets, and to identify and characterize structural analogs of MC540 with superior properties as clinical marrow purging agents. These areas will be investigated in comprehensive fashion, using aqueous solutions and artificial liposomes (Project 1); a natural membrane, the erythrocyte ghost (Project 11); and tumor cells and freshly explanted human marrow cells (Project 111). The techniques employed will range from electron spin resonance spectroscopy to membrane chemistry, fluorescence flow cytometry in vitro and in vivo clonal assays, electron microscopy, and experimental bone marrow transplantation. The proposal is a joint effort of four investigators with expertise in different but complementary areas of photochemistry, photobiology, and phototherapy. The proposed work has implications for the extracorporeal purging of autologous marrow grafts and our understanding of structure-function relationships in merocyanine dyes, the role of activated oxygen species in dye-mediated phototoxicity, and the cellular and molecular events that eventually lead to the death of the target cell.